Hi, this is Erkan Ozgur Yilmaz,
Today we'll talk about how to read MTF charts.
Hi again,
We're together again after a long break,
As I've stated in my previous videos,
because I'm so busy,
we do not get together these days.
But I'm working on to change this.
Although I'm busy, I'll try to do videos this time.
As you know, in my previous videos, especially in lens reviews, I kept talking about these MTF charts.
And I've compared lenses over their MTF charts.
And, a lot of times, I've said that I want to do a video about how to read MTF charts.
And right in this video, we'll exactly do that,
and I'll talk about what MTF charts are how to read them and what you can learn from them, today.
MTF charts, gives information to us about the optical performance of a particular lens.
Over that chart, we can learn, how sharp this lens is,
how is the resolution or the contrast changing towards the edge,
what is the shape of the bokeh or does it have a field curvature or not.
Alright, so why we are using MTF charts and do not compare lenses over photos.
There are a couple of reasons,
First of all, to compare lenses over photos,
you should be able to supply the same conditions,
you should compare them with the same camera,
the same lighting and the same subject, that is.
It is very hard for the manufacturers to create the same photos with the same cameras.
Also, you become dependent to the resolution of the camera.
On the other hand, MTF charts, allows you to measure the lens without limiting the lens to a certain camera or certain resolution.
There are benches like Olaf,
where you can measure the optical performance
of the lens and create the MTF charts.
These benches do not use a digital sensor, they are all working optically.
They allow us to get healthy data.
Also, it makes it easy to compare different brands to each other.
Alright, so MTF charts are much more reasonable to compare lenses,
instead of comparing them over photos.
It is so if you only want to compare optical performances of lenses.
By using MTF charts, first of all,
you can get information about the resolution of the lens.
What is the "resolution of the lens", it is the ability of the lens to reflect high frequency data on to the sensor.
Also, you can get information about the Local Contrast of the lens.
That is, some older lenses produce this blurry photos
where the colors got mixed to each other.
We call this, "low contrast" or "low local contrast".
So, by using MTF charts,
you can get information about the "Local Contrast" of the lens that you are studying.
Thirdly, you get information about the "Astigmatism" of the lens.
So, what is astigmatism?
Astigmatism means that the lens have different resolutions in two different axis.
I know that it doesn't make sense for now, I'll explain it in a minute.
But the biggest outcome of astigmatism is;
If you remember my last lens review video of the 24 f/1.4,
it means that the lens will newer show a point light source as a point if it has "astigmatism".
I'll explain it in detail in a minute.
The fourth thing that you can learn is the "Field Curvature".
That is, the focus will slide towards the edge of the lens,
or let's say it is focusing to another point.
You can read that.
Fiftly, you can learn about the "Focus Shift" of the lens,
that is, the change of focus with the aperture.
I care about 3 of these 5 items.
As I've mentioned, first one is resolution, second one is the local contrast and
the third one is the astigmatism.
These three is enough for me.
I don't study the curves for "Field Curvature" or "Focus Shift".
Let me talk about the details that you can not learn from an MTF chart.
You can not learn anything about the "Distortion" of the lens.
You can not get the colors of the lens,
how accurately the lens is reproducing the colors that is.
You can not get any info about "Vignette".
Again from an MTF chart, you can not learn how much flare does a lens produce.
Let's get in to the technical details of the MTF charts.
Firstly, what does "MTF" mean.
"MTF" means "Modulation Transfer Function", an English name.
or we can directly translate it in to Turkish by "Modülasyon Transfer Fonksiyonu".
That's the meaning.
So what the hell is a "Modulation Transfer Function"?
First, let me talk about what the "Modulation" is.
Modulation is called for a value to change between two different states.
Alright?
If you're working with signal processing, modulation is a wave formed signal.
In photography, the modulation is the name of the visual that is composed from black and white lines.
And "Modulation Transfer Function" in photography,
tells the situation about this black and white stripes
when they entered to the lens and is reflected on to the sensor.
Okay,
For now, it is enough for you to know that the modulation is this black and white stripes.
Okay, let's think like that,
With the optical performance of the lens is changing,
this black and white lines can not be reflected on to the sensor as black and white lines
but as gray and light gray lines.
Because, on the way through the lens these lines gets smoothed,
especially when the lens is not a quality lens.
And when this smoothed lines reflected on the sensor,
what has essentially been done is that
this black and white lines are converted in to smooth lines,
"Transferred" as smooth lines that is.
Let's talk about how the MTF charts are measured.
MTF diagrams are generally measured over a Full Frame sensor.
On a diagonal line positioned at the center to the corner of the sensor,
the "Contrast" of the line stripes
which are placed at 5, 10, 15, 20 and 24 mm distances are measured.
And these measurements are put in a graph as points.
When these points are connected with a line,
you get the MTF chart that is showing the optical performance of the lens.
As for the line density, the 10 lines per millimeter and 30 lines per millimeter are the standard.
But different manufacturers can use other line densities.
Lines along the diagonal are called "Sagittal" lines,
and lines perpendicular to the diagonal are called the "Meridional" lines.
And having different contrasts for these two directions will effect the resolution of the lens.
Generally MTF charts are composed from 4 curves.
The straight ones generally shows the "Sagittal" lines,
and the dashed ones shows the "Meridional" lines.
They generally use two different colors.
Generally they use red and blue,
but some manufacturers use black and blue.
It doesn't matter, they generally put a legend.
But, two different colors tells the performance of different line groups with different densities.
In an MTF chart, the performance of the lens will get better as the lines go up.
Of course, it will get worse as the lines go down, and the lens will produce a more blurry image.
Now, the 10 lpmm curve shows the "Local Contrast" of the lens.
As the 10 lpmm curve go down,
the details in the picture may not be changed,
you still be able to see them,
but all the colors will get mixed.
Alright, you'll get a fogy, dream like look.
The 30 lpmm curve, directly shows the resolution of the lens.
It shows how successfully the lens is reflecting the little details on to the sensor.
Again, as this go up, the lens resolves little details perfectly,
and as it goes down, it means that the lens is not seeing little details.
Generally, in old lenses, although there are rare exceptions, these 10 and 30 lpmm curves are lower than newer lenses.
With these new kind of digital sensors,
of course the resolutions become very high compared to film,
so we started to see lenses with very high optical performances.
The horizontal axis of the graph
gives us information about the performance towards the edge of the photo.
Generally, lenses start with a very high contrast ratio and fall down towards the edges.
The dashed and straight curves on the graph,
that have the same color,
shows the performance of the sagittal and the meridional lines
which are perpendicular to each other.
And this is one thing that you should care about while reading an MTF chart.
It is called "Astigmatism", when the dashed and straight lines become separated.
And astigmatism, shows how much the blurriness is in two different axis as in our eyes.
Let's say that you bought a lens and you're studying the dashed and straight lines as it goes to the edge.
I can directly say this:
If the Sagittals stays on top and the Meridionals gets lower,
the bokeh circles are going to turn in to ellipses,
And these ellipses will form a circle around the photo.
It exists in very old lenses, you'll see a dream like visual,
The bokeh circles will show off as if they are moving.
This one is seen in lenses where the Meridional resolution is very low.
Although it is very rare,
there are lenses with higher meridional resolution and lower sagittal resolution.
Irix is a very good example for that.
In this situation, you'll see that the bokeh of the lens will be line like.
They'll be like lines going from center towards the edges, that is.
The more the lines stay together, the more the quality of the bokeh.
The separation in 10 lpmm lines will tell us about the "bokeh performance",
And the separation of the 30 lpmm lines will tell us about the "coma performance".
And what was the "coma performance": it is the situation of the point light sources to stay as point lights.
If the "coma performance" of the lens you're studying the MTF chart of, is bad,
than the stars in the photo will get in to the shape of a plus or cross sign.
So they can not stay as points.
We prefer the lines to descent together.
So, if the performance of the lens is going to get worse towards the edges,
we prefer it to get worse both in sagittal and meridional lines together.
Alright, we've talked about the three aspects that I care most.
Resolution, contrast, and the astigmatism that explains the bokeh and coma performances.
Now,
yes,
Reading MTF charts is something practical thing.
and you can get experienced by comparing different lenses side by side,
reading their MTF charts and then looking at their sample photos,
then finding the optical imperfections that you saw on the MTF charts on those photos.
Thus,
when you see MTF charts, you'll have an idea about the lens at a glance.
And you can see if it is deserving its retail price at least optically.
As I've mentioned earlier, MTF charts are not telling the whole story.
They give information about some limited aspects of optical performance of the lens.
They don't tell about the vignette of the lens,
or they don't tell about the lateral chromatic aberration of the lens.
they don't tell about the color reproduction,
and, they don't give info about the lens flare.
But you can get a little idea about the shape of the lens flare.
And let's talk about this,
Nowadays, all of the manufacturers release an MTF chart for their lenses.
Different manufacturers will release different MTF charts.
Nikon, for example, gives the 10 lpmm and the 30 lpmm curves, and nothing else.
And they give them at the widest aperture.
And if we're talking about a zoom lens,
they will give two different MTF charts,
one for wide angle and one for tele.
Canon, like Nikon, gives the 10 and 30 lpmm curves,
Other than that, they give the curves for f/8.
Thus, after seeing the widest and
stopped down aperture values,
you can guess the in between values,
they generally follow a linear fashion.
So, you can see how much the performance is getting better.
And, if the lens is not getting better,
then you can guess that
the lens may have a "focus shift" problem.
So, the lens will focus to near or far with the aperture is changing.
Don't mix it with "focus breathing",
"Focus Shift" explains the focus moving with the aperture is stopped down.
Eventhough it is rare, it exists on some lenses.
In Canon's MTF diagrams, you can read that.
Zeiss, gives the 10, 20 and 40 lines per millimeter curves.
So, instead of 30 it shows 20 and 40 lpmm.
Thus, when comparing Zeiss lenses with other brand lenses,
you need to imagine the 30 lpmm in between the 20 and 40 lpmm curves,
or better draw on top of it.
Then you can compare it with other brand lenses.
Because, if you look at the 40 lpmm curve,
the performance of the Zeiss lens may seem worse.
But, you would be wrong by doing that,
because you would compare how it is resolving a finer detail
with a coarser one on the other brand.
Sigma, gives two diagrams called "Geometric" and "Diffraction" MTFs.
The "Geometrical" MTFs is the same with the other brands,
The "Diffraction" MTF is showing the MTF which is measured using the diffraction property of the light.
Because the other brands are not using the "Diffraction" MTF
I don't care about it very much,
I generally look at the "Geometric" one.
On the other hand,
the "Diffraction" MTF gives a much realistic information
about the performance of the lens.
Irix's MTF shows the line frequency in the horizontal axis
instead of the distance on the sensor,
So it starts with 0 lpmm on the left and goes up to 100 lpmm on the right.
They show 4 different curves.
I had to convert this MTF chart to the other form that the other brands are using.
So I can compare it.
But, the MTF diagrams of Irix gives much more information to you.
On this aspect, Irix is much more successful with its MTF diagrams.
The MTF diagrams of Samyang is not different than other brands.
So there is no problem with directly comparing them.
Another thing is how the MTF diagrams are measured.
Most of the manufacturers measure them in the computer.
So it is a virtual data.
It shows the maximum performance that the lens design can get.
That is, if you can produce the lens perfectly the lens will show that MTF behavior.
But, non of the manufacturer will produce lenses matching perfectly to their design.
They will have micron level differences.
In fact, in those simulated MTF diagrams,
you can see the best case scenario, but you never get it.
Manufacturers like Zeiss, measure their MTF diagrams on a real lens.
I find it as a much more valuable information.
But generally, they say not to compare MTF diagrams of different manufacturers.
But if you know these differences, like if it is simulated or real, or what lpmm curve is that,
then you can compare them.
You'll probably get a highly correct verdict about the lenses.
Today, I tried to explain what I know about MTF charts.
Because it is a highly technical subject, it is not very easy to explain it in a video.
I struggled a lot before creating this video and also while shooting it today.
You may have questions,
please use the comment section below and ask anything,
If you like, send the MTF charts so we can talk on them.
So this subject get clear on everybody's mind.
Please like the video, and subscribe to my channel, and share the video.
In the next video,
we will talk about why the aperture numbers are not following a regular series,
but instead are like 1.4, 2, 2.8, 3.5, 4, 5.6 etc.
If you are wondering that please to not miss that video.
See you later,
I thank to my Patrons Özgür Ermutlu and Soner Yurtseven,
because of their huge support to the channel.
I hope, one day, you'll also be one of the supporters of the channel,
See you,
For more infomation >> BEDWARS?!?! Live :3 - Duration: 1:06:00. 
Không có nhận xét nào:
Đăng nhận xét